1. Field of the Invention
The present invention relates to a battery voltage monitoring circuit that monitors the battery voltage of multiple rechargeable batteries connected in series.
2. Description of the Related Art
In recent years, portable devices such as digital cameras include lithium-ion batteries as rechargeable batteries. Lithium-ion batteries are easily affected by overcharge and overdischarge, so that it is necessary to provide a protection circuit for overcharge and overdischarge.
In the case of using multiple lithium-ion batteries (cells) connected in series as a single battery pack, in order for the protection circuit to properly detect the overcharge voltage and overdischarge voltage of each of the lithium-ion batteries, there should be no open circuit in the connection between the batteries and the protection circuit, and if there is an open circuit, such control as the prevention of charge and discharge is necessary. Further, if there is reduction in battery voltage for some reason, it is necessary to prevent subsequent charging, which is unsafe.
A technique is known where a charge and discharge control circuit for a rechargeable battery having two or more cells connected in series has voltage divider circuits connected to the respective cells; the divided voltages are detected in voltage detector circuits; the voltages of the voltage detector circuits change in response to the cells becoming overdischarged or overcharged; their signals are input to a control circuit so that a signal is output to a switch circuit provided in a power supply unit to stop overdischarge or overcharge; and a constant current circuit is added in order to detect the disconnection of the connection of the cells and the charge and discharge control circuit. (See, for example, Japanese Laid-Open Patent Application No. 8-308115.)
Further, a technique is known where in a battery pack monitoring apparatus that detects the presence or absence of an open circuit in interconnects led out from a battery pack having multiple cells connected in series, auxiliary current paths having set resistance values are connected in parallel to the respective cells, and the presence or absence of an open circuit is detected based on a detection voltage at the time of closing switch units for auxiliary current paths. (See, for example, Japanese Laid-Open Patent Application No. 2009-288034.)